As best understood, prior art portable open fault instruments have been of at least two types; those which utilize a constant current or a constant voltage fed through a resistor to the capacitance of an unknown length of cable (or wire pair) where the time to charge that capacitance to a predetermined threshold level is measured and used as an indication of the value of the capacitance, and systems which operate as a capacitance bridge with the unknown capacitance inserted in the test circuit as one leg of that bridge.
Analog meters which utilize the capacitance of the cable or wire pair under test to transfer a precision generated frequency therethrough, as in a coupling capacitor between amplifiers, may have been used in a laboratory environment to make comparable measurements.
Neither system handles "dirty" open circuits well. A "dirty" open circuit, as is well known in the industry, is one which is somewhat less than an infinite impedance open circuit. Many of the problems encountered in the field are of that nature; there is a finite impedance in the so-called "open circuit" which acts to the detriment of transfer of useful communications signals down the line.
The prior art instruments which have been available to the industry for portable field test work have tended to be expensive, of large volume and relatively heavy, some being on the order of fifteen pounds in weight.